An EL lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. The dielectric layer can include phosphor particles or there can be a separate layer of phosphor particles adjacent the dielectric layer. A modern (post-1985) EL lamp is typically made by depositing layers of inks on a substrate, e.g. by roll coating, spraying, or various printing techniques. The techniques for depositing ink are not exclusive, although the several lamp layers are typically deposited in the same manner, e.g. by screen printing. In the context of a thick-film EL lamp, and as understood by those of skill in the art, “inorganic” refers to a crystalline, luminescent material that does not contain silicon or gallium. The term does not refer to the other materials from which an EL lamp is made.
The inks used include a binder, a solvent, and a filler, wherein the filler determines the nature of the ink. As long known in the art, having the solvent and binder for each layer be chemically the same or chemically similar provides chemical compatibility and good adhesion between adjacent layers; e.g., see U.S. Pat. No. 4,816,717 (Harper et al.). It is not easy to find chemically compatible phosphors, dyes, binders, fillers, solvents or carriers and to produce, after curing, the desired physical properties, such as flexibility, and the desired optical properties, such as color and clarity.
EL phosphor particles are typically zinc sulfide-based materials, including one or more compounds such as copper sulfide (Cu2S), zinc selenide (ZnSe), and cadmium sulfide (CdS) in solid solution within the zinc sulfide crystal structure or as second phases or domains within the particle structure. EL phosphors typically contain moderate amounts of other materials such as dopants, e.g., bromine, chlorine, manganese, silver, etc., as color centers, as activators, or to modify defects in the particle lattice to modify properties of the phosphor as desired. The color of the emitted light is determined by the doping levels. Although understood in principle, the luminance of an EL phosphor particle is not understood in detail.
It is known in the art to define a graphic by patterning a lamp layer; i.e., the front electrode, the phosphor layer, the rear electrode, or combinations thereof. The granular nature of the phosphor and the lambertian emission of light necessarily limit the resolution of a graphic. One solution known in the art is to place a graphic layer over an EL lamp. This has the advantage of potentially producing a high resolution display but has the disadvantages of cost and power consumption because one is lighting areas that are masked by the graphic. Using a patterned lamp layer and a graphic layer reduces power consumption but introduces problems of registration with the graphic layer, which increase the cost of the display.
It is known in the art to control field intensity between the electrodes of an EL lamp, and therefore brightness, by adding a dielectric (insulating) layer. U.S. Pat. No. 3,201,633 (Lieb) discloses phosphor regions having a high dielectric constant and other areas of low dielectric constant among the phosphor regions in a single layer. The areas may include light absorbing material. In another embodiment, the phosphor regions are embedded into a layer having a low dielectric constant, producing a reduced thickness portion of the layer between the phosphor and the front electrode. U.S. Pat. No. 5,508,585 (Butt) discloses adding a dielectric layer under the rear electrode to produce a graphic. U.S. Pat. No. 5,686,792 (Ensign, Jr.) discloses using a dielectric layer to elevate interconnects above the level of rear electrodes to make non-luminous interconnects. U.S. Pat. No. 6,411,726 (Pires) discloses a resin layer between the front electrode and the phosphor in an EL lamp. U.S. Pat. No. 7,088,039 (Barnardo et al.) discloses putting a dielectric layer under conductive runs to rear electrodes to reduce unwanted light emission. In a complex display, the routing of interconnects can be difficult and typically limits the design of a display, particularly for concentric, separately lit elements.
Published PCT application WO/2006/072796 (Tyldesley et al.) discloses an “intermediate” or “protective” layer between the ITO and the phosphor in an EL lamp. It is disclosed that “in the prior art display, the barium titanate in the dielectric layer can react with the Indium Tin Oxide (ITO) transparent electrode in some manner in the curing process to turn the ITO slightly yellow. In order to protect the ITO, the protective layer may be arranged to ensure separation of the dielectric material and the substantially transparent electrode during manufacture of the display.” Curiously, it is also disclosed that “the performance of the display may be enhanced by including a small concentration of Barium Titanate in the intermediate layer.” The disclosure of the published application is somewhat mysterious in that the intermediate layer “may” include a dye. It is disclosed that “An example of such a dye is X.” None of this may be a problem for one trying to reproduce the disclosed invention because “the intermediate layer may be one that dissipates or otherwise disappears during the curing process.”
It is known in the art that luminosity is non-linearly proportional to the voltage on an EL lamp. That is, in a perfectly dark chamber with suitable instruments, one could detect light emission from a phosphor at just a few volts. Such extreme conditions are irrelevant to this invention, which is not concerned with esoteric possibilities but is concerned with the perception of light under normal conditions of use for a portable electronic device. Thus, an area is perceived as “off” if a person with unaided normal vision cannot detect light emission under normal circumstances. For example, normal circumstances do not include careful inspection or unusual darkness.
In the market for displays, there is a continuing demand for greater flexibility, greater clarity, lower power consumption, and, especially, lower cost. Taken for granted are many other considerations, such as brightness, environmental stability (can endure high temperature and high humidity or low temperature), electrical stability, and low noise (electrical or acoustic). Such a market is a continuously moving target and difficult to satisfy.
In view of the foregoing, it is therefore an object of the invention to provide an EL panel having high resolution graphics and low cost.
Another object of the invention is to provide an EL panel that simplifies the production of complex graphics for display.
A further object of the invention is to provide an EL panel with minimal stray light emission.
Another object of the invention is to provide an EL panel that provides high resolution graphics without patterning at least one electrode.
A further object of the invention is to provide an EL panel that has low power consumption despite the use of a mask over areas of phosphor.
Another object of the invention is to provide an EL panel that displays graphics and is characterized by simplicity of design, both optically and electrically.
A further object of the invention is to provide an EL panel that displays graphics characterized by fine line geometries.
Another object of the invention is to provide an EL panel that displays high resolution graphics including separately addressable, concentric areas.